Proposed research is directed toward an understanding of certain mechanisms of blood coagulation. (1) Fibrin clot development inherently involves an enzymatic conversion by thrombin of fibrinogen to monomer fibrin, and a set of association reactions which form the three dimensional cross linked fibrin network. Physiological agents can affect particular interactions differentially. Our objective is to prepare pure fibrinogen and to characterize its interaction with thrombin. Following this, the physiological degradation products produced by thrombin and plasmin will be introduced into the system and alteration in clotting characteristics determined. (2) The final stage in the intrinsic and extrinsic systems is the production of thrombin by a system which contains prothrombin (F-II), F-V, F-X, F-VII, tissue lipoprotein and calcium ion (extrinsic) or prothrombin, F-V, F-Xa, platelet lipoprotein and calcium ion (intrinsic). Our objective is to complete procedures for the isolation of prothrombin, F-V, F-X and F-Xa and to use these in conjunction with tissue thromboplastin or platelet factor 3 to reconstitute converting systems. By correlating rates of thrombin production with factor concentrations and with the appearance of conversion intermediates an attempt will be made to reveal the mechanism of conversion. (3) A gel filtration procedure has been developed by which the platelet environment can be rapidly manipulated. This procedure will be applied with the objectives of determining those plasma constituents which are required to preserve normal platelet characteristics, of determining the limits of reversible platelet perturbation, and of exploring the mechanisms by which platelets respond in producing their hierarchy of irreversible responses, particularly of adhesion, cohesion and release.